It will be appreciated that nuclear fuel assemblies must be dependable in order to allow reliable operation of nuclear reactors. Thus, design and construction provisions for such assemblies have been drawn up.
These provisions impose a general framework and minimum criteria which the assembly constructors must take into consideration.
As far as the guide tubes are concerned, the design provisions require verification that they remain stable under axial compression and that the mechanical integrity thereof is not affected during such compression. These criteria are aimed in particular at taking into consideration the axial compression of the guide tubes which results from contact against the upper bearing plate of the core by springs carried by an upper end piece of the assembly.
Although the criteria imposed by the design provisions allow assemblies to be designed with satisfactory reliability, these are merely minimum criteria and it is desirable to develop a method which allows even more reliable assemblies to be designed.
For all that, it is also desirable to reduce the safety margins during design in order to reduce the mass and the cost of the assemblies constructed.